Rapid industrialization leads to increased wastewater discharge encompassing hexavalent chromium (Cr(VI)), which leads to serious environmental problems of toxicity and potential carcinogenicity. Removal of these species is normally carried out by ion-exchange, precipitation, membrane filtration, sorption, photocatalytic reduction, etc. This review mainly focuses on the photocatalytic and photoelectrocatalytic (PEC) reduction of Cr (VI), because of their advantages over other methods such as reduced risk of secondary pollution by non-reduced Cr (VI), no sludge formation, no need for a large amount of chemical reagents, clean and easy installation. The main factors influencing the photocatalytic reduction efficiency of Cr (VI) such as catalyst activity, solution pH, Cr adsorption on the catalyst and additives, are briefly discussed. Finally, a special emphasis is provided to the photoelectrocatalytic (PEC) reduction of Cr (VI).

Progress on the Photocatalytic Reduction Removal of Chromium Contamination.

Development of a new efficient visible-light-driven photocatalyst from SnS2 and polyvinyl chloride

Sodium-doped carbon nitride nanotubes (Na
 x
 -CNNTs) were prepared by a green and simple two-step method and applied in photocatalytic water splitting for the first time. Transmission electron microscopy (TEM) element mapping and X-ray photoelectron spectroscopy (XPS) measurements confirm that sodium was successfully introduced in the carbon nitride nanotubes (CNNTs), and the intrinsic structure of graphitic carbon nitride (g-C3N4) was also maintained in the products. Moreover, the porous structure of the CNNTs leads to relatively large specific surface areas. Photocatalytic tests indicate that the porous tubular structure and Na+ doping can synergistically enhance the hydrogen evolution rate under visible light (λ > 420 nm) irradiation in the presence of sacrificial agents, leading to a hydrogen evolution rate as high as 143 μmol·h−1 (20 mg catalyst). Moreover, other alkali metal-doped CNNTs, such as Li
 x
 -CNNTs and K
 x
 -CNNTs, were tested; both materials were found to enhance the hydrogen evolution rate, but to a lower extent compared with the Na
 x
 -CNNTs. This highlights the general applicability of the present method to prepare alkali metal-doped CNNTs; a preliminary mechanism for the photocatalytic hydrogen evolution reaction in the Na
 x
 -CNNTs is also proposed.

Sodium-doped carbon nitride nanotubes for efficient visible light-driven hydrogen production

Multifunctional magnetic adsorbents containing MnO2 and polyaniline (PANI) with optimized adsorption properties toward heavy metal ions have been developed. In particular, Fe3O4 spheres were chosen as the magnetic core, followed by PANI and MnO2 coating, realizing the formation of Fe3O4/PANI/MnO2 core–shell hybrids. The as-synthesized Fe3O4/PANI/MnO2 core–shell hybrids showed a hierarchical structure with a large surface area and high magnetic saturation value. In comparison with Fe3O4/PANI and Fe3O4/MnO2 core–shell hybrids, Fe3O4/PANI/MnO2 core–shell hybrids displayed the highest adsorption capacity toward heavy metal ions (including Cd(II), Zn(II), Pb(II) and Cu(II)), thanks to the integrated physical and chemical adsorption behaviors resulting from MnO2 inorganic oxide and the PANI polymer. The developed multifunctional Fe3O4/PANI/MnO2 adsorbents synthesized by a facile and economic route are believed to show high potential in environmental remediation for heavy metal removal.

Fe3O4/PANI/MnO2 core–shell hybrids as advanced adsorbents for heavy metal ions

A graphitic carbon nitride (g-C3N4) nanosheet and its nanocomposites have recently attracted increasing interest due to their massive potentials in applications ranging from fluorescence imaging to solar energy conversion. An economical mass-production method for the synthesis of g-C3N4 nanosheets is urgently needed for the application of these intriguing nanomaterials. Here we develop a facile and rapid route to synthesize g-C3N4 nanosheets by using chemical exfoliation followed by extraction and thermal treatment. The feature of this approach lies in its rapid speed with exfoliation time of only about one minute and facile operation free of long-time ultrasonication or stirring, filtration and repeated washing processes to remove the residual acid. Moreover, the method is high-yield and easily upscalable. Meanwhile, the exfoliated g-C3N4 nanosheets exhibit high adsorption capability and photocatalytic activity due to the synergistic effects of large surface area, decreased recombination probability of photoinduced electron–hole pairs and enlarged band gap. This simple and rapid route enables the possibility of large-scale synthesis of g-C3N4 nanosheets with high yield, thus promoting their application in environmental purification and solar energy conversion.

Shuangzhi Liu

Papers

Enhancement of the photocatalytic activity of g-C3N4 via treatment in dilute NaOH aqueous solution

Partially conjugated polyvinyl chloride-modified TiO2 nanoparticles for efficient visible-light-driven photocatalytic reduction of aqueous Cr(VI)

One-step solvothermal preparation of visible light-driven N-modified Nb2O5 photocatalyst using nitric acid as the nitrogen source